It’s a little bit clunky, but you can use GitHub.dev/<repo> on your phone. I do a lot of browsing/quick edits that way, and it works well enough. That said, I also use Working Copy for iOS to sync my notes repo, and will use that sometimes as well.

Personally, I don’t find that I do a lot of “deep writing” on my phone, as opposed to quick recall or in-the-moment scratch padding. So as long as I have a way to access my notes, make quick edits, and then sync them with my laptop (where I’ll do my primary thinking later), then I’m pretty content.

Look into gitjournal if you're storing and editing markdown from git repos: https://gitjournal.io/ It's very nice in my experience.